In the field of packaging machines for products, especially liquid products inside containers of heat sealable material, the operation of welding the material by which the containers are sealed is of fundamental importance. Liquid products may be packed, for example in "sachets" or "envelopes" formed from a composite material based on a thin sheet of aluminium and at least one plastic layer with heat-sealable properties. Usually the sealing operation is carried out by means of a pair of heated bars disposed one opposite the other between which material which is conveniently in the form of a strip unwinding from a bobbin and suitably folded longitudinally so as to present two edges parallel to each other is fed. The bars are pressed towards one another against this material with a force suited to the characteristics of the said material, in phase with the times when the feed line of the material used for forming the containers is stationary, the temperature of the sealing bars and the contact time between the sealing bars and the material to be sealed being synchronised with the operating speed of the packaging machine.
The quality of the sealing, on which the quality of the packaging depends, must be maintained in time with the machine rating, and above all even with the machine working production speeds different from the rating, and where, consequently, there is a variation in one of the elements forming the basis for the regulation of compression force applied by the sealing bars.
For the application of pressure by the sealing bars the use of springs (whose compression force is determined through the manual setting of purpose built adjusting ring nuts) is well known. (See U.S. Pat. No. 3,657,055) This means first of all that in order to vary the compression force applied by such sealing bars it is necessary for the operator to adjust each ring nut directly; this can require the machine to be stopped to allow the operator to carry out the adjustments. Furthermore this method is unsuitable where the machine is used with frequent changes in the production rate because the springs, set for certain operating speed of the machine, if not continually reset, as appropriate for the production rate changes can be the cause of an imperfect sealing.
According to another well known method, adjusting devices for the compression force applied by sealing bars use pneumatically controlled small pistons. (See U.S. Pat. Nos. 3,279,150 and 4,295,922). This method undoubtedly offers the possibility of easier adjustment of pressure of the sealing bars compared to the method using pressure springs; it does however have the inconvenience of not succeeding, with absolute reliability, in maintaining the phase of the operative cycle of the machine especially if there are continuous variations of production speed, with consequent adverse effect on the quality of the sealing.